Polymer crosslinking agents or "crosslinkers" are multi-functional molecules capable of reacting with pendant functional groups on polymers. The use of crosslinkers enables one to increase the molecular weight of the polymer, usually in a second step, and thus improves the properties of the resulting polymer or polymeric film. Most crosslinking reactions are initiated by heating a mixture of the polymer and the crosslinker either neat or in a solvent. Such systems are often referred to as "thermosetting" systems.
Crosslinkers are particularly useful in coating applications due to the fact that the crosslinker enables the use of relatively low molecular weight polymers and resins which are easily handled in solvents. The formulation can subsequently be applied to the substrate and heated, or cured, to give the finished (thermoset) coating. This makes it possible to take advantage of the ease of handling and solubility characteristics of the lower molecular weight resins used in the formulation and subsequently develop the hardness, chemical and solvent resistance, as well as strength/flexibility properties desired in the ultimate coating by the reaction of the crosslinker with the resin during the curing process.
Crosslinkers are becoming increasingly important due to the emphasis on more environmentally acceptable coatings. One major environmental concern in the coatings industry is the amount of organic solvent released during the curing process. This solvent level or Volatile Organic Content (VOC) is of concern due to the role of organic solvents in the development of photochemical smog. For these reasons various governments, including the U.S., are regulating the VOC levels of coating formulations. One way to reduce the amount of solvent necessary in a coating formulation is to reduce the molecular weight of the resin backbone used in the formulation. When this approach is used, however, crosslinking becomes even more critical to the development of the ultimate properties in the cured film. Thus, in these applications the crosslinker enables a more environmentally sound coating formulation.
Properties of Crosslinked Films and Coatings:
A number of properties are desired in a coating in order to impart the desired protection of the object from corrosion and other environmental factors. Some of the protective characteristics that are ultimately desired include the resistance of the coating to various chemicals and solvents, the impact strength of the system, the hardness of the coating and the weather-ability, or resistance of the system to various factors related to environmental exposure.
I) Chemical and Solvent Resistance
In order for a coating to impart adequate protection to the object coated it must be resistant to various chemicals and solvents. If a coating is not resistant to solvents and chemicals, the coating could be removed or the protective integrity compromised by exposure to commonly used materials such as cleaners or gasoline. Since the coating formulation is usually applied in a solvent, development of solvent resistance in the cured film indicates a change in the chemical nature of the coating formulation. This change can be attributed to the crosslinking of the polymer. A commonly used test to assay this property is the methyl ethyl ketone (MEK) rub resistance of the coating. The MEK rub resistance of a coating is often a good diagnostic test for determining the extent of cross-linking in coatings. For most applications, a MEK rub resistance of greater than 175-200 is generally desired.
II) Impact Strength
In order for a coating to be resistant to collisions and other sudden impacts the material must have certain strength characteristics. If a coating does not possess enough strength, impacts and/or collisions will lead to chipping and breaking of the coating which, in turn, compromises the protective integrity of the film. A commonly used test for the impact strength of a coating (ASTM D2794-84) is to drop a weight from various heights on a coated panel and determine the force(in inch-lbs.) required to break the coating. Proper crosslinking can help develop the impact strength of a coating.
III) Hardness
In order for a coating to be resistant to scratching and other such abrasions the coating must possess a certain degree of hardness. This resistance to scratching is often determined by marring the coating with pencils of various hardness and noting which hardness of pencil actually scratches the coating.
Hardness and impact strength often work in opposite directions. This is due to the fact that impact strength reflects both the strength and the flexibility of the polymeric film, while hardness reflects primarily just the strength or rigidity of the film. Thus one often seeks a combination of hardness and flexibility by compensating one of the above characteristics for the other.
The compensation of these two factors is best understood by invoking the theory of crosslink density. If the coating formulation consists of a group of poly-functional (n&gt;2) polymer molecules and crosslinker then the crosslinking process can be thought of as consisting of a series of steps. Initially, the crosslinking reaction consists of intermolecular reactions of various polymer chains. During the initial phase the polymer and crosslinker chains are combining and thus building in molecular weight, but, the mobility of the resulting polymer chains is not greatly restricted. This stage would be characterized by improvement in the chemical resistance, hardness and impact strength of the film. At some point, however, intermolecular reaction is essentially complete and intramolecular reaction becomes significant. At this point, the polymer becomes more rigid due to restriction of the polymer chain mobility by these intramolecular reactions and the resulting coating becomes more brittle. At this stage hardness will improve but the impact strength will decrease, due to the increased rigidity of the polymer network. The balance between flexibility and hardness can be controlled by the amount of crosslinker used, the average functionality of the polymer and crosslinker as well as the chemical structure of the polymer or crosslinker.
IV) Resistance to Atmospheric Exposure (Weathering)
Since many coated objects are exposed to severe weather conditions the performance of the coating under various exposure conditions is very important. Factors which affect the weatherability of the coating include the composition of the polymer and the crosslinker, as well as the degree of crosslinking. A variety of exposure tests are available which enable one to determine the performance of the system to severe conditions.
Crosslinkers Currently Used in the Field:
A large number of crosslinkers are used in various applications. A partial list of the more commonly used functional groups used in crosslinkers include:
Epoxy Compounds PA1 Isocyanates PA1 Amino resins PA1 Unsaturated compounds PA1 (a) a curable acrylic copolymer prepared by the free radical polymerization of from about 1 to 50 weight percent, based on the total weight of monomers, of a monomer compound of formula (I) ##STR2## and one or more monoethylenically unsaturated monomers of a structure other than formula (I) , and PA1 (b) an amino-functional crosslinking agent. PA1 (a) a curable acrylic copolymer prepared by the free radical polymerization of from about 1 to 50 weight percent, based on the total weight of monomers, of a monomer compound of formula (I) ##STR3## and one or more monoethylenically unsaturated monomers of a structure other than formula (I), and PA1 (b) an amino-functional crosslinking agent, with the proviso that said agent is other than methylenediamine, ethylenediamine, or hexylenediamine. PA1 (a) dissolving vinyl ethylene carbonate in an organic solvent to form a solution; PA1 (b) heating said solution to about 60 to about 130.degree. C.; followed by PA1 (c) adding to said solution, one or more mono-ethylenically unsaturated monomers, said monomers being other than vinyl ethylene carbonate, along with a free radical initiator. PA1 (i) acrylic, methacrylic, crotonic, or other unsaturated acids or their esters such as methyl methacrylate, ethyl acrylate, butyl acrylate, propyl acrylate, butyl methacrylate, 2-ethyl hexyl acrylate, dimethyl amino ethyl methacrylate, hydroxy ethyl methacrylate, glycidyl methacrylate, and the like; PA1 (ii) styrene-type monomers such as styrene, alpha-methyl styrene, vinyl toluene, and the like; PA1 (iii) vinyl compounds such as vinyl chloride, vinyl acetate, vinyl proprionate, vinyl 2-ethyl hexanoate, vinyl pivalate, and the like; PA1 (iv) allyl compounds such as allyl alcohol, allyl chloride, allyl acetate, and the like; PA1 (v) other copolymerizable unsaturated monomers such as dimethyl maleate, maleic anhydride, dimethyl itaconate, acrylonitrile, acrylamide, isoprene, butadiene, and the like. PA1 (d) removing unreacted vinyl ethylene carbonate as a vapor under reduced pressure.
These materials take advantage of the reaction of the aforementioned functional groups with various pendant groups on the polymeric backbone. These crosslinkers can be used in combination with other crosslinkers to impart a variety of desired characteristics to the coatings. The use and reactions of these crosslinkers have been reviewed elsewhere. (See, for example, Labana, S. S., in "Encyclopedia of Polymer Science and Engineering", Vol. 4, pp. 350-395.
We have found that 4-ethenyl-1,3-dioxolan-2-one can be copolymerized under solution and emulsion free-radical polymerization conditions with a variety of ethylenically unsaturated monomers to yield cyclic carbonate functional copolymers. These copolymers can be crosslinked with primary amine functional materials to form crosslinked coatings curable under air-drying and force drying conditions. The resulting coatings contain urethane crosslinks, but have been formed without the use of polyfunctional isocyanates.
The homopolymerization of vinyl ethylene carbonate results in a low conversion of monomer to polymer, usually less than 50 percent. Bissinger, et al., J. Am. Chem. Soc., 69, 2955-2961, describe the preparation and homopolymerization of vinyl ethylene carbonate. U.S. Pat. No. 2,511,942 also describes the preparation and homopolymerization of vinyl ethylene carbonate. Seisan-kenkyu, 25(7), 297-299 (1973) describes the polymerization of vinyl ethylene carbonate and subsequent reaction of the homopolymer with butyl amine. JP 62022375 and 62254303 describe the use of the homopolymer of vinyl ethylene carbonate as an electrolyte for lithium batteries.
Seisan-kenkyu, 25(7), 297-299 (1973) also describes the copolymerization of vinyl ethylene carbonate with various polymerizable comonomers and the reaction of a copolymer with styrene with ethylenediamine. U.S. Pat. No. 4,263,418 describes graft copolymers containing vinyl ethylene carbonate as one of the co-monomers used. JP 62254303 and 62022375 also describe the use of copolymers of vinyl ethylene carbonate for use as battery electrolytes.
U.S. Pat. No. 4,772,666 describes the use of polymers containing carbonate and other functional groups in a resin which is crosslinked with either isocyanates or melamine resins to form a coating.